Fix RuntimeError when pretty-printing a dict mutated during iteration - #15472

Summary

Fixes #14541.

Pretty-printing a mapping that mutates while it is being formatted raised:

RuntimeError: dictionary changed size during iteration

The reported trigger is vars(posix) on its first call: a module's
__dict__ populates lazily the first time its attributes are accessed, so it
grows while the pretty-printer is iterating it (which is why it works on every
subsequent call, or after completion has already touched the attributes).

Root cause

_dict_pprinter_factory in IPython/lib/pretty.py iterated the mapping's
live key view and looked each value up again:

keys = obj.keys()
for idx, key in p._enumerate(keys):
    ...
    p.pretty(obj[key])

If anything mutates obj during the loop, iterating the live view raises
RuntimeError.

Fix

Snapshot the items once, up front, and iterate that snapshot:

items = list(obj.items())
for idx, (key, value) in p._enumerate(items):
    ...
    p.pretty(value)

The live mapping is now read exactly once, before any value is formatted, so a
mutation triggered during formatting can no longer break iteration. Capturing
(key, value) pairs also removes the second live obj[key] lookup. Behaviour
for ordinary, non-mutating mappings (including dict, os.environ, and
types.MappingProxyType, which share this factory) is unchanged.

Testing

• Added test_dict_mutated_while_pretty_printing, which formats a dict that
  grows while one of its values is being pretty-printed. It reproduces the
  exact RuntimeError on main and passes with this change.
• pytest tests/test_pretty.py — all tests pass (54).
• ruff check reports no new findings on the changed lines.

Notes

Added a docs/source/whatsnew/pr/ fragment describing the fix.